Setting the clock to the rhythms of the universe

If you ever find yourself needing to know the time, there is no better person on the planet to ask than Jun Ye. A physicist working at JILA, a joint research institute of the University of Colorado and the National Institute of Standards and Technology (NIST), Ye’s group recently built the world’s most advanced and precise clock. 

The device is so accurate that it would neither gain nor lose a second over the age of the universe. It is so precise that, because of the connection between space and time on Earth, if you move it by a millimeter, you can tell the time has changed. “This sounds like science fiction,” adds Ye. “But the clock accuracy is so good now, we can tell within 1 cm where we are with respect to the Earth’s gravitational potential, thanks to Einstein’s general relativity that connects time and gravitational potential.” 

The clock is the culmination of a career developing and refining precise measurement tools with an array of talented scientists. Ye earned his Master’s at the University of New Mexico while also studying in the renowned quantum optics group led by quantum optics theorist (and fellow Quantum West Plenary speaker) Marlan Scully. In the mid-1990s, Ye’s PhD advisor at JILA was John ‘Jan’ Hall. Hall joined NIST in 1961, just after the birth of the laser, and spent most of the rest of his career making lasers increasingly stable. Under Hall’s mentorship, Ye developed the world’s most sensitive instrument for measuring the wavelengths and quantities of light absorbed by different molecules. 

After a two-year postdoctoral stint in trailblazing quantum optics experimentalist Jeff Kimble’s laboratory at the California Institute of Technology, where he developed an optical system to trap individual atoms in a small cavity, Ye was lured back to JILA in 1999, where Hall was still working. “Jan was considering retirement,” recalls Ye. “And so he basically said, ‘Well, since you’re coming back, maybe you can have my lab’.” 

But Hall wasn’t quite done yet. He had recently co-developed a new technology called the optical frequency comb, a type of laser that produces a series of pulses with a very precise delay between them whose spectrum, zoomed in, looks like a discrete rainbow, with sharp spikes of the different colors at precise, evenly spaced intervals — much like the teeth of a comb (hence the name). 

The comb is an extremely accurate ruler for measuring the frequencies of light waves, and Hall was eager to explore its applications. For four years, Ye worked with Hall and colleagues on developing the optical frequency comb and utilizing it in a broad range of spectroscopy applications. But when Hall finally officially retired in 2003 — two years before receiving a share of the 2005 Nobel Prize in Physics for his optical frequency comb work — Ye sought a new challenge. Combining his experience of trapping and cooling atoms with his knowledge of optical frequency combs, Ye wanted to build optical atomic clocks with laser-cooled strontium atoms (where the tick of the clock comes from the atoms’ electrons undergoing optical transitions between two stable orbitals) in order to surpass the accuracy of incumbent atomic clocks. The technique turned out to be a great success. “It has been a very exciting journey for the last 20 years where we have been able to improve optical atomic clock precision by five orders of magnitude,” Ye says. 

Ye’s Plenary talk at Quantum West could easily focus on the many groundbreaking discoveries he and his collaborators have made to reach this exquisite level of timekeeping accuracy. But he feels this would be a wasted opportunity. “I want to focus on what are we doing right now,” he says. “The atomic clock is the core of a quantum revolution where we can perform very precise quantum state engineering, bring many atoms together to work as a team of quantum particles.” 

So far, Ye’s group has brought together tens of thousands of strontium atoms and controlled their interaction at the quantum level to build extremely precise optical atomic clocks. His current work aims to add to the number of atoms in these systems significantly. “We want to scale the systems up to 1 million atoms. We will need to understand all the intricacy of atomic interactions in a many-body setting and, in parallel, we may need to now put entanglement into the quantum systems so that we can hide away some of the quantum noise,” he says. “And that’s really where quantum information science is heading — can we really build a system with so many quantum particles working together, keeping their entanglement, keeping their quantum coherence to use this to take a measurement or process information?” 

In one sense then, Ye’s clocks are like microscopes peering down into the quantum world and revealing all sorts of important insights for various applied quantum fields, like quantum information processing, quantum communication, and quantum sensing. In another sense, the clocks are like telescopes. “We want to see how far we can push the clock precision to the next two orders of magnitude or better because then we can start to measure the gravitational effect across a quantum mechanical wave function,” Ye explains. “And that happens to also be the point where you can use the clock itself to listen to gravitational waves, using time to directly detect these spacetime ripples.”

But perhaps the most exciting role for atomic clocks in the coming decade will be as probes of the fundamental nature of the universe. “In the end, it’s the spacetime fabric that the clocks are measuring, but you need quantum physics to make those clocks,” says Ye. Therefore, the clocks are like a bridge between the macroscopic world governed by Einstein’s theory of general relativity and the microscopic world governed by quantum mechanics. With sufficient precision, this could mean atomic clocks play a pivotal role in unmasking the nature of dark matter, challenging Einstein’s conception of determinism in motion, and finally understanding the connection between, and perhaps even eventually reconciling  our currently incompatible theories of the macroscopic and microscopic worlds. 

To realize this potential, Ye’s atomic clocks — at present, consisting of myriad mirrors, miniature lenses, optoelectronic devices, and lasers arranged on a 4 x 12 ft optical table — need to be made more practical so that they can be used in the field and eventually space. And to do this will require innovations in miniaturizing the clocks’ components. “The engineering itself is going to require a lot of creativity and novel ideas,” Ye says. “How do we use integrated photonics to shrink the system to a much smaller size? There will be a lot of interesting science and technology coming out of those efforts.” Ye hopes to connect with experts in integrated photonics at Photonics West who can help him make miniaturized optical atomic clocks a reality. 

And, although cautious of making predictions, knowing the talent that resides within the scientific community makes Ye optimistic that the necessary precision, accuracy, and portability will be achieved to realize optical atomic clocks’ full potential in the next 5–10 years. “Past performance cannot guarantee future success, but let me just say that over the last five years, we achieved a factor of nearly 100 accuracy improvement,” he says. “It may take a lot of effort, we may need to develop another new interesting technology, and there will be surprises (that I know for sure), but I don’t see any fundamental limiting factor for why we cannot achieve another factor of 100,” he says.